1. Field of the Invention
The present invention relates to systems and methods for signal processing, and in particular to a system and method for reducing intermodulation distortion.
2. Description of the Related Art
It is sometimes advantageous to combine a plurality of signals, each having its own carrier, to create a multi-carrier signal. Unfortunately, multi-carrier signals often have high peak-to-average power ratios. When such a multi-carrier signal is subjected to a peak—limiting channel (such as a non-linear power amplifier) the signals (and its components) undergo significant spectral distortion, leading to both in-band and out-of-band interference. This interference can cause significant degradation in system performance.
Methods have been developed to decrease the interference resulting from applying signals having high peak-to-average power ratios to nonlinear amplifiers (including those with a clipping or saturation point).
One technique to reduce the effect of the non-linearity is to back off the output signal level from the amplifier's saturation point. While effective, this reduces the transmitted signal power and overall power efficiency, thereby significantly increasing the complexity of the thermal control and power subsystems.
A second technique is to use feed-forward and feedback amplifier techniques. Feed forward techniques provide excellent correction and bandwidth, but again with additional hardware and significant DC power and thermal control design. Feedback techniques are inherently narrow band, and therefore less suitable for wide bandwidth applications.
A third technique involves pre-distorting the input signal. Representative examples of the application of this technique are described in Chow, JS., Bingham, J. A. C., and Flowers, M. S., “Mitigating Clipping Noise in Multi-Carrier Systems,” Communications, 1997. ICC 97 Montreal, at 1997 IEEE International Conference on Communications: Towards the Knowledge Millennium, Vol. 2 , 1997, pp. 715-719; O'Neill, R., Lopes, L. B., “Envelope Variation and Spectral Splatter in Clipped Mulit-Carrier Signals”, Personal, Indoor and Mobile Communications, 1995. PRMC'95. ‘Wireless: Merging on into the Information SuperHighway’., Sixth International Symposium on, Volume: 1, Pages 71-75, 1995; and Pauli, M. and Kuchenbecker, P., “on the Reduction of the Out-of-Band Radiation of OFDM-Signals,” Conference Record, 1998 IEEE International Conference on Communications, Vol. 3, 1998, pp1304-1308, which are incorporated by reference herein. However, while the techniques described in these publications are effective in reducing distortion levels across the entire frequency band, they typically do not reduce distortion to sufficiently low levels. What is needed is a system and method for reducing intermodulation distortion that produces very low distortion levels in sub-bands of interest.
The present invention satisfies that need.